The Aptus Non tithe grade means that no tithe is required to be paid. Most classes of Aptus Non worlds either provide service to the Imperium in a unique way (such as Adeptus MechanicusForge Worlds, or Adeptus Astartes Homeworlds) or are worlds which would simply not be taxed, such as lifeless Dead Worlds, or worlds unable to produce or provide anything of value.[1]

However, Space Marine Chapters have to give 5% of their gene-seed as a "tithe", for various purposes. It is imperative that the gene-seed is examined for signs of taint or mutation, and if the particular chapter is wiped out, a portion of their gene-seed remains, ensuring that one day the chapter might be re-founded.[1]